Small Active Area Research Articles

Photoresponse characteristics of bulk gallium nitride schottky barrier metal-semiconductor-metal ultraviolet photodetectors

In this work, bulk gallium nitride (GaN) Schottky barrier metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors (PDs) were fabricated, and their key performance metrics such as photoresponse, photosensitivity, gain, external quantum efficiency, detectivity, noise equivalent power, rise time, and fall time were thoroughly evaluated. The Schottky barrier was formed using platinum (Pt) metal with two different sizes of interdigitated electrode on the bulk GaN samples.The effects of mask size on the photoresponse characteristics were investigated. The Pt/bulk GaN MSM UV PDs exhibited outstanding performance under 365 nm UV light. At a bias voltage of 5 V, the PDs with smaller active areas (0.238 cm²) achieved a responsivity of 13.6 mA/W and a fast rise time of 174 ms, while those with larger active areas (0.412 cm²) demonstrated a responsivity of 2.62 mA/W and a rise time of 286 ms. Even at 0 V bias, these devices showed responsivities of 1.34 µA/W and 3.40 µA/W for the small and large masks, respectively, indicating that they possess self-powered device capabilities.

Optimal performance of silicon nanowire solar cells under low sunlight concentration and their integration as bottom cells in III–V multijunction systems

Nanostructured silicon solar cells are designed to minimize costs through reduced material usage while enhancing power conversion efficiency via superior light trapping and shorter charge separation distances compared to traditional planar cells. This study identifies the optimal conditions for nanoimprinted silicon nanowire (SiNW) solar cells to achieve maximum efficiency under low sunlight concentration and evaluates their performance as bottom cells in III–V multijunction solar cell systems. The findings indicate that the SiNW solar cell reaches its peak performance at a concentration factor of 7.5 suns and a temperature of 40°C or lower. Specifically, the absolute conversion efficiency under these conditions is 1.05% higher than that under unconcentrated light. Compared to a planar silicon solar cell under identical conditions, the SiNW solar cell exhibits a 3.75% increase in conversion efficiency. Additionally, the SiNW single-junction solar cell, when integrated in series with a commercial lattice-matched InGaP/GaAs dual-junction solar cell, was tested under unconcentrated sunlight, specifically at one-sun, global air mass 1.5 condition, to assess its viability in one-sun multi-junction solar cell applications. The results suggest that a III–V upper subcell with a smaller active area than that of the SiNW subcell is optimal for maximizing current production, which is favorable to the cost reduction of the device. This hybrid configuration is particularly advantageous for terrestrial applications, such as electric vehicles, which demand lightweight, high-performance multijunction solar cell devices. Although the weight reduction of the characterized SiNW solar cell with a full silicon substrate compared to its planar solar cell counterpart is 1.8%, recommendations to increase this reduction to as much as 64.5% are discussed to conclude this paper.

Radiation Damage on Silicon Photomultipliers from Ionizing and Non-Ionizing Radiation of Low-Earth Orbit Operations.

Silicon Photomultipliers (SiPMs) are single photon detectors that gained increasing interest in many applications as an alternative to photomultiplier tubes. In the field of space experiments, where volume, weight and power consumption are a major constraint, their advantages like compactness, ruggedness, and their potential to achieve high quantum efficiency from UV to NIR makes them ideal candidates for spaceborne, low photon flux detectors. During space missions however, SiPMs are usually exposed to high levels of radiation, both ionizing and non-ionizing, which can deteriorate the performance of these detectors over time. The goal of this work is to compare process and layout variation of SiPMs in terms of their radiation damage effects to identify the features that helps reduce the deterioration of the performance and develop the next generation of more radiation-tolerant detectors. To do this, we used protons and X-rays to irradiate several Near Ultraviolet High-Density (NUV-HD) SiPMs with small areas (single microcell, 0.2 × 0.2 mm2 and 1 × 1 mm2) produced at Fondazione Bruno Kessler (FBK), Italy. We performed online current-voltage measurements right after each irradiation step, and a complete functional characterization before and after irradiation. We observed that the main contribution to performance degradation in space applications comes from proton damage in the form of an increase in primary dark count rate (DCR) proportional to the proton fluence and a reduction in activation energy. In this context, small active area devices show a lower DCR before and after irradiation, and we propose light or charge-focusing mechanisms as future developments for high-sensitivity radiation-tolerant detectors.

Thin film AgBiS2 solar cells with over 10 % power conversion efficiency enabled by vapor-assisted solution process treatment

Recently, thin film photovoltaic solar energy has grown rapidly with new materials for achieving high conversion efficiency and long-term stability. Especially, silver bismuth sulfide (AgBiS2) nanocrystal-based quantum-dots have emerged as viable absorber for cost-effective photovoltaic. However, a thick AgBiS2 quantum-dots layer always faces the dilemma of entailing the charge carrier collection and trap-assisted recombination. Here we show a vapor-assisted solution process to fabricate high crystallinity submicron-grain AgBiS2 films. The resultant devices with small active area (0.06 cm2) achieved a record-breaking power conversion efficiency of 10.20 % and large active area (1.00 cm2) achieved an efficiency of 9.53 % under 100 mW cm−2 standard AM 1.5 global sunlight simulation, both of which are the highest reported for thin film AgBiS2 solar cells to date. Notably, the solar cells based on submicron-grain AgBiS2 thin films showed an exceptional durability, maintaining over 94 % of the initial PCE for 3000 h in ambient air, along with excellent stability under extreme conditions, including exposure to 85 °C and 85 % relative humidity.

Printing and Coating Techniques for Scalable Organic Photovoltaic Fabrication.

Within recent years, there has been an increased interest towards organic photovoltaics (OPVs), especially with their significant device performance reaching beyond 19% since 2022. With these advances in the device performance of laboratory-scaled OPVs, there has also been more attention directed towards using printing and coating methods that are compatible with large-scale fabrication. Though large-area (>100 cm2) OPVs have reached an efficiency of 15%, this is still behind that of laboratory-scale OPVs. There also needs to be more focus on determining strategies for improving the lifetime of OPVs that are suitable for scalable manufacturing, as well as methods for reducing material and manufacturing costs. In this paper, we compare several printing and coating methods that are employed to fabricate OPVs, with the main focus towards the deposition of the active layer. This includes a comparison of performances at laboratory (<1 cm2), small (1-10 cm2), medium (10-100 cm2), and large (>100 cm2) active area fabrications, encompassing devices that use scalable printing and coating methods for only the active layer, as well as "fully printed/coated" devices. The article also compares the research focus of each of the printing and coating techniques and predicts the general direction that scalable and large-scale OPVs will head towards.

Photoactive polymer coatings for antibacterial applications

Because bacteria can readily induce illnesses in the human body, research into new bactericidal approaches remains relevant to the global community. Photoactive materials (e.g., photocatalysts) provide an environmentally-friendly method for killing bacteria with light. By means of light-mediated reactive oxygen generation, photocatalysts are capable of disrupting bacterial membranes. Much research on antibacterial surfaces has focused on planar substrates or physisorbed coatings, which often provide small active surface areas and limited surface-to-volume ratios, and thereby create a need for new design strategies to incorporate photocatalysts onto more scalable supports. To this end, this contribution describes the synthesis of particle-tethered polymer brush coatings, crosslinked with the organic photocatalyst tetrakis(4-carboxyphenyl)porphyrin (TCPP). Successful engineering on SiO2 surfaces was confirmed using X-ray photoelectron spectroscopy, tensiometry, and ellipsometry. Singlet oxygen (1O2) generation was confirmed through dimethyl sulfoxide oxidation experiments under visible (blue) light and bactericidal activity against Escherichia coli was verified through live/dead assays and cell viability tests.

Ultrafast room-temperature construction of hierarchical hollow rodlike nanoarrays based on amorphous NiFeCo layered double hydroxide toward enhanced oxygen evolution reaction

Designing and constructing non-noble metal electrocatalysts with high catalytic activity for oxygen evolution reaction (OER) is a significant challenge for energy storage and conversion. Transition-metal-based layered double hydroxides (LDHs) are the most efficient OER catalysts. However, their catalytic performance and commercial application are limited by their small electrochemical active areas and poor electronic conductivity. Herein, hollow hierarchical rodlike nanoarrays on nickel foam assembled with amorphous NiFeCo-LDH nanosheets were prepared using a metal–organic framework, ZIF-67, as a sacrificial template by a facile and ultrafast ion-exchanged reaction at room temperature. The as-prepared electrocatalyst possessed a large specific surface area (397 m2/g), unique structure, fast charge transfer and abundant active sites, and thus displayed a superior performance for OER with a low overpotential of 202 mV at 10 mA/cm2, a Tafel slope of 47.9 mV/dec and long-term durability in 1.0 M KOH electrolyte. This research affords a facile strategy to rationally design high activity electrocatalysts of multi-metallic LDHs for OER.

Research on Dual Fuel Combustion Characteristics of PFI Natural Gas +DI Gasoline in Large Cylinder Diameter Engine

A 3D simulation model of a single cylinder of a natural gas engine was established. PFI natural gas DI gasoline was used to control the time matching between injection and ignition, so that gasoline could form a small active stratified area near the spark plug. The combustion characteristics under different matching conditions were studied, and finally compared with pure natural gas. It is found that gasoline can be used in large cylinder diameter natural gas engine in a small range, which can accelerate the combustion of natural gas fuel, shorten the time used by CA0-CA5 by 77.6%, shorten the time used by CA5-CA90 by 26.5%, and increase the indicated thermal efficiency of natural gas engine by 1.4%.

High efficiency graphene-silicon hybrid-integrated thermal and electro-optical modulators.

Graphene modulators are considered a potential solution for achieving high-efficiency light modulation, and graphene-silicon hybrid-integrated modulators are particularly favorable due to their CMOS compatibility and low cost. The exploitation of graphene modulator latent capabilities remains an ongoing endeavour to improve the modulation and energy efficiency. Here, high-efficiency graphene-silicon hybrid-integrated thermal and electro-optical modulators are realized using gold-assisted transfer. We fabricate and demonstrate a microscale thermo-optical modulator with a tuning efficiency of 0.037 nm mW-1 and a high heating performance of 67.4 K μm3 mW-1 on a small active area of 7.54 μm2 and a graphene electro-absorption modulator featuring a high speed data rate reaching 56 Gb s-1 and a low power consumption of 200 fJ per bit. These devices show superior performance compared to the state of the art devices in terms of high efficiency, low process complexity, and compact device footage, which can support the realization of high-performance graphene-silicon hybrid-integrated photonic circuits with CMOS compatibility.

Boosting the Output Performance of the MoS2 Monolayer-Based Piezoelectric Nanogenerator by Artificial Dual Strain Concentration.

Piezoelectric nanogenerators (PENGs) with molybdenum disulfide (MoS2) monolayers have been intensively studied owing to their superior mechanical durability and stability. However, the limited output performance resulting from a small active area and low strain levels continues to pose a significant challenge that should be overcome. Herein, we report a novel strategy for the epoch-making output performance of a PENG with a MoS2 monolayer by adopting the additive strain concentration concept. The simulation study indicates that strain in the MoS2 monolayer can be initially augmented by the wavy structure resulting from the prestretched poly(dimethylsiloxane) (PDMS) and is further increased through flexural deformation (i.e., bending). Based on these studies, we have developed concentrated strain-applied PENGs with MoS2 monolayers. The wavy structures effectively applied strain to the MoS2 monolayer and generated a piezoelectric output voltage and current of around 580 mV and 47.5 nA, respectively. Our innovative approach to enhancing the performance of PENGs with MoS2 monolayers through the artificial dual strain concept has led to groundbreaking results, achieving the highest recorded output voltage and current for PENGs based on two-dimensional (2D) materials, which provides unique opportunities for the 2D-based energy harvesting field and structural insight into how to improve the net strain on 2D materials.

A 28-GHz eight-element phased-array receiver front-end with compact size in 65-nm CMOS technology for 5G new radio

This article presents an 8-element 24–30-GHz phased-array receiver front-end in 65-nm CMOS technology for the millimeter-wave fifth-generation mobile network (5G). The phased-array receiver front-end is based on the radio-frequency (RF) phase-shifting architecture using RF vector-sum phase shifters with 5-bit phase resolution. The designed front-end can be easily expanded for use in large-scale phased arrays due to its compact size, low manufacturing costs, reasonable power consumption and low complexity. The post-layout simulations show a maximum single-element average gain of 12.85–15.7 dB across 5-bit phase states, with an associated noise figure (NF) of 6.15–7.5 dB and an input third-order intercept point (IIP3) value between –16.5 and –15.4 dBm. The simulated RMS gain and phase errors are less than 0.31 dB and 3°, respectively over 32 phase states and are constant over a 6-bit gain control. The designed eight-element phased-array receiver front-end has a total power consumption of 256±2.5 mW from a 1.2-V supply and occupies a small active area of 0.62 mm2.

Fe and Mo Co-modulated Coral-like Nickel Pyrophosphate in-situ Derived from Nickel-Foam for Oxygen Evolution.

The highly active catalyst for the oxygen evolution reaction (OER) is critial to achieving high efficiency in hydrogen generation from water splitting. Direct converting nickel foam (NF) into nickel-based catalysts has attracted intensive interest due to the tight interaction of the catalysts to the substrate surface. However, the catalytic performances are still far below expectation because of the problems of low catalyst amount, thin catalyst layer and small active area caused by the method limitation. Herein, we develop a Fe3+-induced synthesis strategy to transform the NF surface into a thicker catalyst layer. In addition to the excellent conductivity and high stability, the as-prepared FeMo-Ni2P2O7/NF catalysts expose more active sites and facilitate mass transfer due to their thicker catalyst layer and highly dense coral-like micro-nano structure. Furthermore, the Mo, Fe co-modulation optimizes the adsorption free energies of the OER intermediates, boosting the catalytic activities. Its catalytic activity is among the highest, and it exhibits a small Tafel slope of 34.71 mV/dec and a low overpotential of 161 mV for delivering current density of 100 mA/cm2 in all of the reported Ni-based catalysts. The present strategy can be further used in other catalysts design for energy storage and conversion.

An On-/Off-Time Sensing-Based Load-Adaptive Mode Control of Triple Mode Buck Converter for Implantable Medical Devices.

Wireless power transfer (WPT) technology applied to implantable medical devices (IMDs) significantly reduces the need for battery replacement surgery health conditions. This paper presents an on-/off-time sensing-based load-adaptive mode control of triple mode buck converter for implantable medical devices; the converter can adjust the control mode for low power consumption and achieve high power conversion efficiency (PCE) under a small active area. The three modes in the proposed system are the pulse width modulation (PWM), pulse frequency modulation (PFM), and ultra-low power (ULP) modes. The on-time sensor can be used to adjust the system from PWM to PFM modes, and the off-time sensor can be used to adjust the system from PFM to ULP modes. It is fabricated using TSMC 0.18 µm CMOS technology. The input voltage lies in the range 2.2-5.0 V, the output voltage is 1.8 V, and the load current lies in the range 0.05-200 mA (x4000). The experimental results demonstrate the seamless mode transition under the step up/down load transient response. The peak PCE is approximately 94.3% at the 80 mA and the minimum PCE is approximately 65.4% within the load current range.

Millimeter-scale active area superconducting microstrip single-photon detector fabricated by ultraviolet photolithography.

The effective and convenient detection of single photons via advanced detectors with a large active area is becoming significant for quantum and classical applications. This work demonstrates the fabrication of a superconducting microstrip single-photon detector (SMSPD) with a millimeter-scale active area via the use of ultraviolet (UV) photolithography. The performances of NbN SMSPDs with different active areas and strip widths are characterized. SMSPDs fabricated by UV photolithography and electron beam lithography with small active areas are also compared from the aspects of the switching current density and line edge roughness. Furthermore, an SMSPD with an active area of 1 mm × 1 mm is obtained via UV photolithography, and during operation at 0.85 K, it exhibits near-saturated internal detection efficiency at wavelengths up to 800 nm. At a wavelength of 1550 nm, the detector exhibits a system detection efficiency of ∼5% (7%) and a timing jitter of 102 (144) ps, when illuminated with a light spot of ∼18 (600) µm in diameter, respectively.

Comet P/2021 HS (PANSTARRS) and the Challenge of Detecting Low-activity Comets

Jupiter-family comet (JFC) P/2021 HS (PANSTARRS) only exhibits a coma within a few weeks of its perihelion passage at 0.8 au, which is atypical for a comet. Here we present an investigation into the underlying cause using serendipitous survey detections and targeted observations. We find that the detection of the activity is caused by an extremely faint coma being enhanced by the forward scattering effect owing to the comet reaching a phase angle of ∼140°. The coma morphology is consistent with sustained, sublimation-driven activity produced by a small active area, ∼700 m2, one of the smallest values ever measured on a comet. The phase function of the nucleus shows a phase coefficient of 0.035 ± 0.002 mag deg−1, implying an absolute magnitude of H = 18.31 ± 0.04 and a phase slope of G = − 0.13, with color consistent with typical JFC nuclei. Thermal observations suggest a nucleus diameter of 0.6–1.1 km, implying an optical albedo of 0.04–0.23, which is higher than typical cometary nuclei. An unsuccessful search for dust trail and meteor activity confirms minimal dust deposit along the orbit, totaling ≲108 kg. As P/2021 HS is dynamically unstable, similar to typical JFCs, we speculate that it has an origin in the trans-Neptunian region and that its extreme depletion of volatiles is caused by a large number of previous passages to the inner solar system. The dramatic discovery of the cometary nature of P/2021 HS highlights the challenges of detecting comets with extremely low activity levels. Observations at high phase angle, where forward scattering is pronounced, will help identify such comets.

Preparation of Perovskite Solar Cells in the Air: Degradation Mechanism and Prospects on Large‐Area Fabrication†

Comprehensive SummaryThe preparation of perovskite solar cells (PSCs) in the air environment has attracted the attention of numerous experimenters due to its low preparation cost and the possibility of commercialization. Although the power conversion efficiency (PCE) of PSCs has increased rapidly and exceeded 25%, which is comparable to commercial polysilicon solar cells, most certified or reported high‐efficiency perovskite solar cells are still confined to glove boxes or relatively small active areas in the air environment due to moisture, oxygen, high temperature, and ultraviolet (UV) factors. In this review, the factors that lead to perovskite degradation are reviewed, and the appropriate strategies for manufacturing high‐efficiency and stable perovskite solar cells under environmental conditions are summarized to help the technology be commercialized for high‐quality, stable, and large‐area perovskite thin films in the air.

Electrical passivation of III-V multijunction solar cells with luminescent coupling effect

Perimeter recombination is one of the causes identified for having a nonuniform luminescent coupling effect in III-V multijunction solar cells. A potential solution could be the electrical passivation of the multijunction solar cell perimeter. To test this hypothesis, electrical passivation of InGaP/GaAs/Ge 3-junction solar cells was done by the atomic layer deposition of thin Al 2 O 3 films. Perimeter passivation of a 3-junction solar cell relatively increased current collection measured at luminescent coupling and direct subcell excitation by as much as 11.4% and 29.8%, respectively. Meanwhile, the current homogeneity improved relatively by as much as 7.9% after electrical passivation. At 1-sun global standard illumination (AM 1.5G), 0.2% absolute conversion efficiency increase was achieved. Therefore, electrical passivation of fully working III-V multijunction solar cells is a non-invasive, post-treatment process that can recover losses due to surface defects caused by oxidation. • Post-treatment passivation for fully working multijunction solar cell. • Larger conversion efficiencies in multijunction solar cells with smaller active areas. • Spatial current uniformity improvement after perimeter passivation. • Reduction of shunt leakage current in a subcell made of indirect bandgap material. • Alleviate lateral resistance effects by electrical passivation.

High comprehensive performance superconducting nanowire single photon detector

Superconducting nanowire single photon detector (SNSPD) has been widely used in quantum communication, quantum computing and other fields because of its excellent timing jitter and response speed. However, due to the mutual restraint of the technical parameters of SNSPD nanowires, there are technical challenges to further improve the comprehensive performance of SNSPD, and thus limiting its application on a large scale. Combining high detection efficiency with high timing performance is still an outstanding challenge. In this work, we report the SNSPD with 12-μm small active area, which has high speed, high efficiency, low jitter and broadband absorption. Au/SiO&lt;sub&gt;2&lt;/sub&gt; membrane cavity, which is determined by finite element analysis simulation, is used to widen the optical response bandwidth. And it is easier to process and improve the alignment accuracy at the same time. The flat substrate is more conducive to the growth of superconducting thin films, so flattening process is introduced. Device package is also optimized to match smaller detector. Self-aligned packaging makes optical alignment more convenient and time-saving. Special optical fibers with small mode-field diameters can reduce the negative effect of the detector on optical coupling. The detector can achieve a maximum SDE of 82% at the central wavelength of 1310 nm and the temperature of 2.2 K, and the SDE of more than 65% in the wavelength range of 1200–1600 nm, with DCR of 70 cps. The detector also exhibits a count rate of 40 MHz@3 dB and a timing jitter of 38 ps, which is significantly improved compared with 23-μm active area detector. Furthermore, the minimum timing jitter of 22 ps can be obtained by using cryogenic amplifier readout. In this work, high comprehensive performance detector is developed, which provides an important technical reference for practical and product SNSPD.

Preferential perovskite surface-termination induced high piezoresponse in lead-free in situ fabricated Cs3Bi2Br9-PVDF nanocomposites promotes biomechanical energy harvesting.

Lead-free halide perovskites have gained immense popularity in photovoltaic and energy harvesting applications because of their excellent optical and electrical attributes with minimal toxicity. We synthesized composite films of lead-free Cs3Bi2Br9 perovskite embedded in the polyvinylidene fluoride (PVDF) matrix and have investigated their piezoelectric energy harvesting. Five PVDF@Cs3Bi2Br9 composite films were fabricated with varying wt% of the perovskite in the PVDF. The composite with a 4 wt% of the perovskite shows 85% activation of the electroactive β-phase of PVDF. Additionally, this composite exhibits a maximum polarisation of ∼0.1 μC cm-2 and the best energy storage density of ∼0.8 mJ cm-3 at an applied field of ∼16 kV cm-1 among all the synthesized composites. A nanogenerator fabricated using 4 wt% loading in the composite film produced an instantaneous output voltage of ∼40 V, an instantaneous current of ∼4.1 μA, and a power density of ∼17.8 μW cm-2 across 10 MΩ resistance when repeatedly hammered by the human hand. The nanogenerator is further employed to light up several LEDs and to charge capacitors with a small active area demonstrating significant promise for prospective wearables and portable devices and paving the way for high-performance nanogenerators using lead-free halide perovskites. Density functional theory calculations were performed to understand the interaction of the electroactive phase of the PVDF with different perovskite surface terminations to unravel the various interaction mechanisms and their ensuing charge transfer properties.

Automatic Gbps Receiver for Mobile Device in Beam-Steered Infrared Light Communication System

This work presents a beam-steered infrared light communication (BS-ILC) system that enables high data rate wireless optical transmission to individual mobile users. The BS-ILC system comprises arrayed waveguide grating router-based optical antennas, camera-based user localization, and optical receivers (each integrated with an infrared tag). A high-speed photodetector has typically a small active area to minimize its junction capacitance, but this small size will result in a very limited field of view (FoV), which is detrimental to an optical wireless system aimed to provide some degree of mobility to users. Thus, the desired BS-ILC system requires precise and automatic alignment to establish a high-performance optical link between users and optical antennas. In this work, we propose an optical wireless receiver that can automatically align its orientation with respect to the optical antenna to ensure the light angle of incidence within the receiver’s FoV. The automatic receiver consists of a motorized actuator and an optimization control algorithm. The actuator controls the orientation of the receiver, and the optimization algorithm is developed to accelerate the searching process of the actuator. In an indoor laboratory system setup, the amount of less than 200 ms has been demonstrated to establish gigabit-per-second optical link, and seamless transmission has also been realized for the mobile receiver while on the move with the FoV of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm 0.6^{\circ }$</tex-math></inline-formula> .